Single burst single satellite beacon localization

Abstract

A method and devices are disclosed, for localization of a radio beacon at a remote receiver in the framework of a satellite system. Such satellite system could be Cospas-Sarsat, for Search and Rescue of people, ships and aircraft in distress, and particularly its MEOSAR (Medium Earth Orbit Search and Rescue) segments: DASS/GPS, SAR/Galileo and SAR/Glonass; said beacon is typically one of a PLB (Personal Locator Beacon) or EPIRB (Emergency Position Indicating Radio Beacon) or ELT (Emergency Locator Beacon); and said remote receiver is typically a MEOLUT (Medium Earth Orbit Local User Terminal) base station. Present art MEOSAR localization is based on Time measurements and Frequency measurements on signals emitted by radio beacons, relayed by satellites and detected at a MEOLUT; however since the exact time of transmission of the beacon is unknown at the MEOLUT, Time Difference of Arrival (TDOA) equations are applied. The present invention however, discloses that by carefully configuring the time of transmission at said beacon, even without directly communicating that specific time to the MEOLUT, Time of Arrival (TOA) equations could be applied at the MEOLUT enabling enhanced localization accuracy and/or fewer satellites in view required to localize the beacon. In particular, localization is enabled even upon a single burst emitted by the beacon and relayed to the MEOLUT by a single satellite.

Claims

1. A method for localization of a radio beacon at a remote receiver, via a first satellite payload and a second satellite payload, comprising the steps of: Configuring said first satellite payload to broadcast a first signal at a known epoch (tx.sub.e), then periodically repeat first signal transmissions at fixed time intervals; Configuring said radio beacon to detect and dynamically select at least one of said first signals and transmit a second signal a predefined delay (T.sub.1) after detecting said first signal; Enabling a second satellite payload to relay said second signal to said remote receiver; Configuring said remote receiver to determine the location coordinates of said beacon (x, y, z) up to a certain ambiguity, even without knowing said dynamic selection, based on: said first (x.sub.1, y.sub.1, z.sub.1) and second (x.sub.2, y.sub.2, z.sub.2) satellite payloads position and said remote receiver position (x.sub.m, y.sub.m, z.sub.m), and a difference between time at which said relayed second signal is detected at said remote receiver (tr.sub.m) and time at which the first satellite payload transmits the first signal (tx.sub.e), accounting for said predefined delay (T.sub.1).

2. The method according to claim 1, further removing at least part of said ambiguity by information acquired or considered at said remote receiver.

3. The method according to claim 1, wherein said first and second satellite payloads are mounted on same satellite.

4. The method according to claim 1, further configuring said remote receiver to: a. Detect and record the frequency of arrival (FOA) of said relayed second signal; b. Determine the beacon transmission frequency; c. Resolve the beacon coordinates at least in two dimensions (2D).

5. The method according to claim 1, configuring said beacon to transmit said second signal at least twice, and configuring said remote receiver to resolve the beacon coordinates at least in 2D.

6. The method according to claim 1, wherein at least two satellites carry a payload as said second payload, and said remote receiver to resolve the beacon coordinates at least in 2D.

7. A radio beacon for localization via satellites, comprising: a receiver coupled to a transmitter; said receiver configured to detect and dynamically select at least one of first signals broadcast by a first satellite payload at a known epoch (tx.sub.e) and periodically afterwards at fixed time intervals, and said transmitter configured to transmit a second signal a predefined delay (T.sub.1) after detecting said first signal; enabling a second satellite payload to relay said second signal to a remote receiver; and enabling a remote receiver to determine the location coordinates of said beacon (x, y, z) up to a certain ambiguity, even without knowing said dynamic selection, based on: said first (x.sub.1, y.sub.1, z.sub.1) and second (x.sub.2, y.sub.2, z.sub.2) satellite payloads position and said remote receiver position (x.sub.m, y.sub.m, z.sub.m), and a difference between time at which said relayed second signal is detected at said remote receiver (tr.sub.m) and time at which the first satellite payload transmits the first signal (tx.sub.e), accounting for said predefined delay (T.sub.1).

8. The beacon according to claim 7, enabling its localization at said remote receiver at least in two dimensions (2D), upon a single second signal burst transmission, a single satellite carrying both first and second payloads, and a further Frequency of Arrival (FOA) measurement of said relayed second signal.

9. The beacon according to claim 7, wherein said receiver is a Global Navigation Satellite System (GNSS) receiver and said epoch is synchronized with said first satellite payload clock.

10. The beacon according to claim 7, further configured to transmit said second signal at least twice, enabling said remote receiver determining the location coordinates of said beacon at least in 2D.

11. The beacon according to claim 7, configured to transmit further signals similar to said second signal, wherein the transmission time of said further signals is substantially synchronized with Universal Time Coordinated (UTC).

12. A Medium Earth Orbit Local User Terminal (MEOLUT) for localization of radio beacons, comprising an RF module and a processor; said RF module configured to detect signals transmitted from satellites and said processor configured to determine the position of radio beacons; wherein a first satellite payload is configured to broadcast a first signal at a known epoch (tx.sub.e) and periodically afterwards at fixed time intervals; and a beacon configured to detect and dynamically select at least one of said first signals and transmit a second signal a predefined delay (T.sub.1) after detecting said first signal; a second satellite payload relaying said second signal to the MEOLUT; and said MEOLUT configured to determine the coordinates (x, y, z) of said beacon up to a certain ambiguity, even without knowing said beacon dynamic selection, based on: said first (x.sub.1, y.sub.1, z.sub.1) and second (x.sub.2, y.sub.2, z.sub.2) satellite payloads position and said MEOLUT position (x.sub.m, y.sub.m, z.sub.m), and a difference between time at which said relayed second signal is detected at the MEOLUT (tr.sub.m) and time at which the first satellite payload transmits the first signal (tx.sub.e), accounting for said predefined delay (T.sub.1).

13. The MEOLUT according to claim 12, further configured to remove at least part of said ambiguity based on additional information.

14. The MEOLUT according to claim 12, further configured to determine the frequency of arrival (FOA) of said relayed second signal, determine the beacon transmission frequency, and resolve the beacon coordinates at least in two dimensions (2D).

15. The MEOLUT according to claim 12, configured to determine the beacon coordinates at least in 2D, upon a single second signal burst transmission and a single satellite carrying both first and second payloads.

16. The MEOLUT according to claim 12, further configured to detect at least another relayed second signal, and resolve the beacon coordinates at least in 2D.

17. The MEOLUT according to claim 12, further configured to detect said second signal relayed by at least two satellites, and resolve the beacon coordinates at least in 2D.

18. A Global Navigation Satellite System (GNSS) receiver configured to be coupled to a transmitter for remote time synchronization; said receiver configured to detect first signals broadcast by a first satellite payload at a known epoch (tx.sub.e) and periodically afterwards at fixed time intervals, enabling said coupled transmitter to dynamically select at least one of said first signals and transmit a second signal a predefined delay (T.sub.1) after detecting said first signal; wherein a second satellite payload enabled to relay said second signal to a remote receiver; and enabling a remote receiver determining the transmission time of said second signal up to a certain ambiguity, even without knowing said dynamic selection, based on: said first (x.sub.1, y.sub.1, z.sub.1) and second (x.sub.2, y.sub.2, z.sub.2) satellite payloads position and said remote receiver position (x.sub.m, y.sub.m, z.sub.m), and a difference between time at which said relayed second signal is detected at said remote receiver (tr.sub.m) and time at which the first satellite payload transmits the first signal (tx.sub.e), accounting for said predefined delay (T.sub.1).

19. The GNSS receiver according to claim 18, comprising an I/O terminal coupled to the transmitter, said terminal configured to indicate detection time of said first signal or the GNSS clock or the Universal Time Coordinated (UTC).

20. The GNSS receiver according to claim 18, enabling said remote receiver determining the location of said transmitter based on the calculated transmission time of said second signal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other characteristics and advantages of the invention will be better understood through the following illustrative and non-limitative detailed description of preferred embodiments thereof, with reference to the appended drawings, wherein:

(2) FIG. 1 illustrates a present art concept for Beacon Localization based on TOA. Three Space Vehicles (satellites) SVi, i=1, 2, 3, are shown, at known positions (x.sub.i, y.sub.i, z.sub.i), each SV detecting at tr.sub.i a signal emitted at tx from a beacon at unknown (x, y, z) position. The TOA (Time of Arrival) measurement at each satellite defines a sphere on which the beacon should be placed, however for simplicity the picture depicts a circle, and these circles are shown to intersect at a unique point, at which the beacon is placed. At the bottom of the picture, the three navigation equations that describe this trilateration method are presented (C=the speed of light):
[(xx.sub.1).sup.2+(yy.sub.1).sup.2+(zz.sub.1).sup.2]=CTOA.sub.1=C(tr.sub.1tx)
[(xx.sub.2).sup.2+(yy.sub.2).sup.2+(zz.sub.2).sup.2]=CTOA.sub.2=C(tr.sub.2tx)
[(xx.sub.3).sup.2+(yy.sub.3).sup.2+(zz.sub.3).sup.2]=CTOA.sub.3=C(tr.sub.3tx)

(3) FIG. 2 illustrates the present art concept of 2D Hyperbolic Localization based on TDOA. Three Space Vehicles SVi, i=1, 2, 3, are shown, at known positions (x.sub.i, y.sub.i, z.sub.i), each SV detecting at tr.sub.i a signal emitted from a beacon at unknown (x, y, z) position. The TDOA (Time Difference of Arrival) measurement of each pair of satellites defines a hyperbole (two sides symmetric over the y axis) on which the beacon should be placed, and two of these hyperboles are shown to intersect at a unique point, at which the beacon is placed. At the bottom of the picture, the two navigation equations that describe this method are presented (C=the speed of light):
[(xx.sub.1).sup.2+(yy.sub.1).sup.2][(xx.sub.2).sup.2+(yy.sub.2).sup.2]=C*TDOA.sub.12=C*(tr.sub.1tr.sub.2)
[(xx.sub.1).sup.2+(yy.sub.1).sup.2][(xx.sub.3).sup.2(yy.sub.3).sup.2]=C*TDOA.sub.13=C*(tr.sub.1tr.sub.3)

(4) FIG. 3 illustrates a present art concept of Beacon Localization based on FOA. A single Space Vehicle SV.sub.1 is shown, at a known position (x.sub.1, y.sub.1, z.sub.1) orbiting around the earth, at known velocity Vsat, detecting a Doppler shift f of a signal emitted at frequency f, from a beacon at unknown (x, y, z) position. The angle between the satellite movement direction and the beacon is defined by the Doppler equation at the bottom of the picture: (C=the speed of light):
Vsat*cos()/C=f/f

(5) FIG. 4 illustrates TOA Measurement Synchronized with GPS Clock. The diagram at the upper part of the picture shows the transmitted beacon signal vs. GPS TIME and the diagram at the bottom part of the picture shows the detection of said beacon signal (relayed by a satellite, not shown) at the MEOLUT, again vs. same GPS TIME scale. Both the beacon and MEOLUT are shown to simultaneously detect the 1PPS pulse of the GPS, and the beacon is shown to transmit exactly at one of these 1 PPS instants. The MEOLUT detection time is referred to the last 1PPS pulse, and the time difference equals to the distance travelled between beacon and MEOLUT divided by the speed of light.

(6) FIG. 5 illustrates a TOA Configuration According to Present Invention. Two satellites: SV.sub.1 and SV.sub.2 are shown, at known positions (x.sub.1, y.sub.1, z.sub.1) and (x.sub.1, y.sub.1, z.sub.1) respectively; a beacon marked by a triangle icon, shown at unknown position (x, y, z), and a MEOLUT (base station) marked by a trapeze icon placed at a known position (x.sub.m, y.sub.m, z.sub.m). SV.sub.1 is shown to transmit a first signal detected at the beacon, the beacon is shown to transmit a second signal relayed by SV.sub.2 and detected at the MEOLUT. A time scale shown below illustrates that the first signal is transmitted at tx.sub.e, and T.sub.1 after been detected at the beacon the second signal is transmitted, reaching the MEOLUT (via SV.sub.2) at tr.sub.m.

(7) An equation printed at the bottom of the picture represents the TOA measurement related to the illustrated configuration; the left side of the equation represents the time at which the signals travel (and delayed) from SV.sub.1 to the MEOLUT via the beacon and SV.sub.2, these multiplied by the speed of light (C); the right side of the equation represents the total distance travelled based on the position coordinates of SV.sub.1, SV.sub.2, the beacon and the MEOLUT, according to the Pythagorean theorem:
C*[tr.sub.mtx.sub.eT.sub.1)=S1+S2+S3=[(xx.sub.1).sup.2+(yy.sub.1).sup.2+(zz.sub.1).sup.2]+[(xx.sub.2).sup.2+(yy.sub.2)+(zz.sub.2).sup.2]+[(x.sub.mx.sub.2).sup.2+(y.sub.my.sub.2).sup.2+(z.sub.mz.sub.2).sup.2]

(8) FIG. 6 illustrates a TOA Measurement According to a 1.sup.st Embodiment of the Present Invention. Shown are two satellite payloads: GNSS and SAR, a beacon and a MEOLUT; the distance made by the first signal travelling between the GNSS satellite payload and the beacon is marked S1; the distance made by the second signal travelling between the beacon and the SAR satellite payload is marked S2; and the distance made by the relayed second signal travelling between the SAR satellite payload and the MEOLUT is marked S3. Further below, a horizontal time scale is depicted, showing the GPS TIME, in 1 ms steps. The rising edge of each 1 ms clock tick is marked by a short vertical line on said time scale and the rising edge of each 1 s clock tick is marked by a bit longer and thicker vertical line. On that time scale, the transmission time of the GNSS satellite payload is marked as tx.sub.e, explicitly shown to be aligned with a 1 s GPS TIME clock tick, at N*sec wherein N is a natural number. Also on that time scale, the detection time at the MEOLUT is marked as tr.sub.m, earlier than the (N+1)*sec clock tick; both tr.sub.m and N*sec are acquired and measured at the MEOLUT. Also on the time scale, the delay at the beacon between detection of first signal and transmission of second signal is marked T.sub.1, and the delay at the SAR satellite payload in relaying the second signal is assumed zero.

(9) The equation printed at the bottom of the picture expresses the TOA measurement:
C*[tr.sub.mtx.sub.eT.sub.1)=S1+S2+S3

(10) The above equation represents the non-ambiguous TOA calculation made at the MEOLUT, assuming that the MEOLUT can specifically determine: tx.sub.e and tr.sub.m and T.sub.1.

(11) FIG. 7 illustrates a TOA Measurement According to a 2.sup.nd Embodiment of the Present Invention. Shown are two satellite payloads: GNSS and SAR, a beacon and a MEOLUT; the distance made by the first signal travelling between the GNSS satellite payload and the beacon is marked S1; the distance made by the second signal travelling between the beacon and the SAR satellite payload is marked S2; and the distance made by the relayed second signal travelling between the SAR satellite payload and the MEOLUT is marked S3. Further below, a horizontal time scale is depicted, showing the GPS TIME, in 1 ms steps. The rising edge of each 1 ms clock tick is marked by a short vertical line on said time scale and the rising edge of each 1 s clock tick is marked by a bit longer and thicker vertical line. On that time scale, the transmission time of the GNSS satellite payload is marked as tx.sub.e, explicitly shown to be aligned with a 1 s GPS TIME clock tick, at N*sec wherein N is a natural number. Also on that time scale, the detection time at the MEOLUT is marked as tr.sub.m, earlier than the (N+1)*sec clock tick; both tr.sub.m and N*sec are acquired and measured at the MEOLUT. Also on the time scale, the delay at the beacon between detection of first signal and transmission of second signal is marked T1, wherein T1=n*ms and n is a natural number, and the delay at the SAR satellite payload in relaying the second signal is assumed zero.

(12) The equation printed at the bottom of the picture expresses the TOA measurement:
C*(tr.sub.mtx.sub.en*ms)=S1+S2+S3
The equation above represents an ambiguous TOA calculation made at the MEOLUT, assuming that the MEOLUT can determine tx.sub.e and tr.sub.m, yet n is unknown.

(13) FIG. 8 illustrates a TOA Measurement According to a 3.sup.rd Embodiment of the Present Invention. Shown are two satellite payloads: GNSS and SAR, a beacon and a MEOLUT; the distance made by the first signal travelling between the GNSS satellite payload and the beacon is marked S1; the distance made by the second signal travelling between the beacon and the SAR satellite payload is marked S2; and the distance made by the relayed second signal travelling between the SAR satellite payload and the MEOLUT is marked S3. Further below, a horizontal time scale is depicted, showing the GPS TIME, in 1 ms steps. The rising edge of each 1 ms clock tick is marked by a short vertical line on said time scale. On that time scale, the transmission time of the GNSS satellite payload is marked as tx.sub.e, explicitly shown to be aligned with a 1 ms GPS TIME clock tick. Also on that time scale, the detection time at the MEOLUT is marked as tr.sub.m, just after and next to a 1 ms tick of the GPS TIME clock, marked (tx.sub.e+n*ms) i.e. exactly n*ms after tx.sub.e; both tr.sub.m and (tx.sub.e+n*ms) are acquired and measured at the MEOLUT (though n might be unknown). Also on the time scale, the delay at the beacon between detection of first signal and transmission of second signal is marked T.sub.1, and the delay at the SAR satellite payload in relaying the second signal is assumed zero.

(14) The equation printed at the bottom of the picture expresses the TOA measurement:
C*[tr.sub.m(tx.sub.e+n*ms)+n*msT.sub.1]=S1+S2+S3

(15) The equation above represents another ambiguous TOA calculation made at the MEOLUT, assuming that the MEOLUT can determine [tr.sub.m(tx.sub.e+n*ms)], perhaps also T.sub.1, yet n is unknown.

(16) FIG. 9 illustrates the Beacon Block Diagram according to the present invention, comprising two main blocks: a GPS receiver and a UHF transmitter. The GPS receiver is depicted at the left upper part, coupled to a GPS (L-band) antenna, and providing 3 outputs: position (Lat/Lon) and two timing signals: the PRN correlation pulse and the 1 PPS/1 KPPS pulse.

(17) The transmitter block is framed by a dashed line, comprising 5 sub blocks: TCXO (Temperature Compensated Crystal Oscillator), micro-processor, PSK modulator, Carrier frequency generator and power amplifier. The TCXO is the master clock of the beacon, generating a basic frequency of 12.68875 MHz which is multiplied or divided to generate the RF frequency of 406.040 MHz and bit rate of 400 bps for a standard 144 bits message communicated in every burst. The processor generates that message and controls the beacon timing. The PSK modulator transforms the digital bits to PSK (Phase Shift Keying) signals modulating the RF carrier, then amplified to 5 watts by the power amplifier and coupled to the UHF antenna. The PRN correlation pulse is routed to the PSK modulator via a T.sub.1 delay block to show that it controls the timing of the PSK output, i.e. the timing of burst transmission, yet practically this can be done through the micro-processor, and the same applies to the 1 PPS/1 KPPS signal output from the GPS receiver.

(18) FIG. 10 illustrates the MEOLUT Block Diagram according to COSPAS-SARSAT MEOLUT PERFORMANCE SPECIFICATION AND DESIGN GUIDELINES C/S T.019 Issue 1 Dec. 2015. The MEOLUT is divided to five main blocks: Antennas RF subsystems, Antenna management, Reception, Processing, and MEOLUT management; two external interfaces are also shown: to Networked MEOLUT(s) and to MCC.

DETAILED DESCRIPTION

(19) The above examples and description have of course been provided only for the purpose of illustration, and are not intended to limit the invention in any way. As will be appreciated by the skilled person, the invention can be carried out in a great variety of ways, employing more than one technique from those described above, all without exceeding the scope of the invention.

(20) The present invention discloses a method for localization of a radio beacon at a remote receiver, via a first satellite payload and a second satellite payload, comprising the steps of: a. Configuring said first satellite payload to broadcast a first signal at a known epoch; b. Configuring said radio beacon to: Determine the detection time of said first signal; Transmit a second signal a predefined delay after said detection time; c. Configuring said second satellite payload to relay said second signal; d. Configuring said remote receiver to: Record the detection time of said relayed second signal, and said known epoch; Determine self-position, the position of said first satellite payload and the position of said second satellite payload; Enable expressing the location coordinates of said beacon based on said time records, said predefined delay, and said determined positions.

(21) The disclosed method further comprises the steps of: a. Configuring said first payload to periodically repeat said epoch and correspondingly said first signal transmissions at fixed time intervals; b. Configuring said radio beacon to dynamically select at least one of said first signals detection time after which said second signal is transmitted; c. Configuring said remote receiver to enable expressing the location coordinates of said beacon up to a certain ambiguity, even if said dynamic selection is unknown at the remote receiver.

(22) FIG. 5 illustrates a TOA Configuration According to the Present Invention. Two satellites: SV.sub.1 and SV.sub.2 are shown, at known positions (x.sub.1, y.sub.1, z.sub.1) and (x.sub.1, y.sub.1, z.sub.1) respectively; assumable, a first payload is mounted on SV1 and a second payload mounted on SV2; a beacon marked by a triangle icon, shown at unknown position (x, y, z), and a MEOLUT (base station) marked by a trapeze icon placed at a known position (x.sub.m, y.sub.m, z.sub.m). SV.sub.1 is shown to transmit a first signal detected at the beacon, the beacon is shown to transmit a second signal relayed by SV.sub.2 and detected at the MEOLUT. A time scale shown below illustrates that the first signal is transmitted at tx.sub.e (said known epoch) and T.sub.1 (said predefined delay) after been detected at the beacon the second signal is transmitted, reaching the MEOLUT (relayed by SV.sub.2) at tr.sub.m. An equation printed at the bottom of the picture, represents the TOA measurement related to the illustrated configuration; the left side of the equation represents the time at which the signals travel (and delayed) from SV.sub.1 to the MEOLUT via the beacon and SV.sub.2, these multiplied by the speed of light (C); the right side of the equation represents the total distance travelled based on the position coordinates of SV.sub.1, SV.sub.2, the beacon and the MEOLUT, according to the Pythagorean theorem. This equation already marked in the disclosure as [eq.1]:
C*[tr.sub.mtx.sub.eT.sub.1)=S1+S2+S3=[(xx.sub.1).sup.2+(yy.sub.1).sup.2+(zz.sub.1).sup.2]+(xx.sub.2).sup.2+(yy.sub.2)+(zz.sub.2).sup.2]+[(x.sub.mx.sub.2).sup.2+(y.sub.my.sub.2).sup.2+(z.sub.mz.sub.2).sup.2]

(23) As a skilled person may appreciate, [eq.1] expresses the location coordinates (x, y, z) of the beacon based on: 1. the time records tr.sub.m and tx.sub.e 2. the predefined delay (T.sub.1) 3. the determined positions of SV1 (x.sub.1, y.sub.1, z.sub.1), SV2 (x.sub.2, y.sub.2, z.sub.2), and the MEOLUT (x.sub.m, y.sub.m, z.sub.m).

(24) The 3 positions can be determined based on present art methods. For example, navigation satellites constantly report their orbital parameters (elements), enabling remote receivers to determine the satellite position at any time with great accuracy. The MEOLUT position is obviously easy to determine with an embedded GNSS receiver.

(25) The predefined delay (T.sub.1) employed by the beacon can be fixed and communicated to the MEOLUT in advance; It can even be configured to be zero.

(26) The time records: tr.sub.m is measured at the MEOLUT, and tx.sub.e is also communicated to the MEOLUT, directly or preferably indirectly. The indirect way disclosed in the present invention is to refer, at both extreme sides of the combined communication link illustrated in FIG. 5, i.e. at SV1 and the MEOLUT, to a common global clock, which according to a preferred embodiment of the present invention is associated with the GPS TIME or UTC, and furthermore, agree upon discrete epochs at which the transmission may start. Such are for example the rising edge of the 1PPS or 1 KPPS of the GPS TIME clock, or clocks synchronized thereto.

(27) According to a first embodiment of the present invention the first signal is transmitted exactly every 1 s. Further, the beacon is a Personal Locator Beacon (PLB) embedded with a GNSS receiver configured to detect said first signal by correlation with the GPS PRN spread spectrum sequence and detecting some bits of the navigation message, and transmit short bursts every 47.5-52.5 s according to the Cospas-Sarsat standard, to be relayed by SAR satellites and detected by a MEOLUT. Preferably, the satellite serving the beacon are mounted with both a GNSS and a SAR payload, as planned for the MEOSAR, and expected to be fully operational by 2018, based on DASS/GPS, SAR/Galileo and SAR/Glonass constellations. According to this first embodiment, the beacon, when detecting only 1-2 satellites, and is due to transmit a distress burst, transmits the burst exactly T.sub.1=0 s after a successful PRN correlation associated with the rising edge of bit number 1+50n in any subframe of the navigation message, wherein n=0, 1, . . . 5. Since there are 300 bits per subframe transmitted at 50 bps, this means associating the burst timing with the 1 s epoch of the GNSS payload. When the MEOLUT detects the burst originally emitted by the beacon and relayed by the SAR satellite, it assumes that this burst was transmitted immediately upon detecting at the beacon the GNSS signal broadcast at the 1 s epoch. It is quite standard in Cospas-Sarsat to define the transmission time instant of a burst aligned with the rising edge of its 25.sup.th message bit, which is the first information bit immediately after the synchronization frame, and so it is preferably configured here.

(28) FIG. 6 illustrates a TOA Measurement According to a first Embodiment of the Present Invention. Shown are two satellite payloads: GNSSbroadcasting navigation signals, and SARbent pipe relay; a beacon and a MEOLUT; the distance made by the first signal travelling between the GNSS satellite payload and the beacon is marked S1; the distance made by the second signal travelling between the beacon and the SAR satellite payload is marked S2; and the distance made by the relayed second signal travelling between the SAR satellite payload and the MEOLUT is marked S3. Further below, a horizontal time scale is depicted, showing the GPS TIME, in 1 ms steps. The rising edge of each 1 ms clock tick is marked by a short vertical line on said time scale and the rising edge of the 1 s ticks is marked by a bit longer and thicker vertical line. On that time scale, the transmission time of the GNSS satellite payload is marked as tx.sub.e, explicitly shown to be aligned with a 1 s GPS TIME clock tick, N*sec wherein N is a natural number. Also on that time scale, the detection time at the MEOLUT is marked as tr.sub.m, earlier than a (N+1)*sec tick, which is logical since in 1 s the signal travels 300,000 Km and it is not likely to occur with satellites orbiting just 20,000 Km above the earth. Thus, both tr.sub.m and tx.sub.e=N*sec are acquired and measured at the MEOLUT unambiguously. Also on that time scale, the delay at the beacon between detection of first signal and transmission of second signal is marked T1, and the delay at the SAR satellite payload in relaying the second signal is assumed to be zero.

(29) The equation printed at the bottom of the picture expresses the TOA measurement:
C*[tr.sub.mtx.sub.eT.sub.1)=S1+S2+S3; which is a variant of the already defined [eq.1].

(30) Since the MEOLUT can determine unambiguously tr.sub.m and tx.sub.e=N*sec and T.sub.1, the left side of [eq.1] is completely known, and the right side of that equation, as already discussed, includes the unknown (x, y, z) coordinates of the beacon, and the known coordinates of both satellites and the MEOLUT, so [eq.1], as claimed: Enable expressing the location coordinates of said beacon based on said time records, said predefined delay, and said determined positions.

(31) According to a second embodiment of the present invention the first signal is transmitted exactly every 1 s. Further, the beacon is a Personal Locator Beacon (PLB) embedded with a GNSS receiver configured to detect said first signal by correlation with the GPS PRN spread spectrum sequence and detecting some bits of the navigation message, and transmit short bursts every 47.5-52.5 s, according to the Cospas-Sarsat standard, to be relayed by SAR satellites and detected by a MEOLUT. Preferably, the satellites serving the beacon are mounted with both a GNSS and a SAR payload. According to this second embodiment, the beacon, when detecting only 1-2 satellites, and is due to transmit a distress burst, transmits the burst exactly T.sub.1=n*ms after a successful PRN correlation corresponding to the 1 s epoch, wherein n is a natural number and is selected so the time difference between consecutive bursts will be pseudo-random as required by Cospas-Sarsat specifications. When the MEOLUT detects the burst originally emitted by the beacon and relayed by the SAR satellite, it assumes that this burst was transmitted n*ms after the GPS TIME 1 s epoch was detected at the beacon, wherein n is a natural number yet specifically unknown at the MEOLUT.

(32) FIG. 7 illustrates a TOA Measurement According to a 2.sup.nd Embodiment of the Present Invention. Shown are two satellite payloads: GNSS and SAR, a beacon and a MEOLUT; the distance made by the first signal travelling between the GNSS satellite payload and the beacon is marked S1; the distance made by the second signal travelling between the beacon and the SAR satellite payload is marked S2; and the distance made by the relayed second signal travelling between the SAR satellite payload and the MEOLUT is marked S3. Further below, a horizontal time scale is depicted, showing the GPS TIME, in 1 ms steps. The rising edge of each 1 ms clock tick is marked by a short vertical line on said time scale and the rising edge of each 1 s clock tick is marked by a bit longer and thicker vertical line. On that time scale, the transmission time of the GNSS satellite payload is marked as tx.sub.e, explicitly shown to be aligned with a 1 s GPS TIME clock tick, at N*sec wherein N is a natural number. Also on that time scale, the detection time at the MEOLUT is marked as tr.sub.m, earlier than the (N+1)*sec clock tick; both tr.sub.m and N*sec are acquired and measured at the MEOLUT. Also on the time scale, the delay at the beacon between detection of first signal and transmission of second signal is marked T.sub.1, wherein T.sub.1=n*ms and n is a natural number, and the delay at the SAR satellite payload in relaying the second signal is assumed zero.

(33) The equation printed at the bottom of the picture expresses the TOA measurement:
C*(tr.sub.mtx.sub.en*ms)=S1+S2+S3; which is also a variant of the already defined [eq.1].

(34) As in the first embodiment, the right side of that equation includes the unknown (x, y, z) coordinates of the beacon, and the known coordinates of both satellites and the MEOLUT. Regarding the left side of the equation, the MEOLUT can determine tr.sub.m and tx.sub.e=N*sec, however not n (at least on first iteration). At this point the MEOLUT may assess the value of n, for example assuming that S1+S2+S3=3*(average distance between satellite and earth surface), which is about 3*20,000 Km for GNSS satellites, and select the closest n which satisfies [eq.1] under that assumption; further, other values on n could be tried and reject those that are in contrast with system parameters or measurements or operational conditions, such as the satellite orbit, satellite footprint, the detected C/N.sub.0, etc. So finally, it could be possible to determine n at the MEOLUT, hence n could be also serve as a parameter communicated from the beacon to the MEOLUT, e.g. encoding the ID of the first payload satellite, i.e. the GNSS satellite to which the beacon burst was synchronized. Obviously, the MEOLUT can use this information to compile [eq.1].

(35) According to a third embodiment of the present invention the first signal is transmitted exactly every 1 ms. Further, the beacon is a Personal Locator Beacon (PLB) embedded with a GNSS receiver configured to detect said first signal by correlation with the GPS PRN spread spectrum sequence, and transmit short bursts every 47.5-52.5 s, according to the Cospas-Sarsat standard, to be relayed by SAR satellites and detected by a MEOLUT. Preferably, satellites serving the beacon are mounted with both a GNSS and a SAR payload. According to this third embodiment, the beacon, when detecting only 1-2 satellites, and is due to transmit a distress burst, transmits the burst exactly T.sub.1 after a successful PRN correlation. When the MEOLUT detects the burst originally emitted by the beacon and relayed by the SAR satellite, it assumes that this burst is associated with a GPS 1 ms epoch, though the specific 1 ms epoch is unknown at the MEOLUT.

(36) FIG. 8 illustrates a TOA Measurement According to a 3.sup.rd Embodiment of the Present Invention. Shown are two satellite payloads: GNSS and SAR, a beacon and a MEOLUT and the distances S1, S2, S3 as described above. Further below, a horizontal time scale is depicted, showing the GPS TIME, in 1 ms steps. The rising edge of each 1 ms clock tick is marked by a short vertical line on said time scale. On that time scale, the transmission time of the GNSS satellite payload is marked as tx.sub.e, explicitly shown to be aligned with a 1 ms GPS TIME clock tick. Also on that time scale, the detection time at the MEOLUT is marked as tr.sub.m, just after and next to a 1 ms tick of the GPS TIME clock, marked (tx.sub.e+n*ms) i.e. exactly n*ms after tx.sub.e; both tr.sub.m and (tx.sub.e+n*ms) are acquired and measured at the MEOLUT (though n is not necessarily known). Also on the time scale, the delay at the beacon between detection of first signal and transmission of second signal is marked T1, and the delay at the SAR satellite payload in relaying the second signal is assumed zero.

(37) The equation printed at the bottom of the picture expresses the TOA measurement:
C*[tr.sub.m(tx.sub.e+n*ms)+n*msT.sub.1]=S1+S2+S3

(38) As in the second embodiment, the right side of that equation includes the unknown (x, y, z) coordinates of the beacon, and the known coordinates of both satellites and the MEOLUT. Regarding the left side of the equation, the MEOLUT can determine [tr.sub.m(tx.sub.e+n*ms)], however not n. Similarly to the second embodiment, the MEOLUT may assess the value of n, and reject those that are in contrast with some system parameters or measurements or operational conditions.

(39) Further according to said first, second and third embodiments of the present invention, the MEOLUT configured to detect the frequency of arrival (FOA) of the relayed beacon bursts, determine the beacon transmission frequency and resolve the beacon coordinates at least in 2D. As already described, the combination of [eq.1]+[eq.2]+[eq.3] enable 2D localization upon a single burst and single satellite (mounted with both GNSS and SAR payloads).

(40) Obviously, if two SAR satellites are in common view by both the beacon and MEOLUT, and the MEOLUT detects a beacon burst relayed by those two satellites, then two independent [eq.1] can be composed at the MEOLUT, enabling 2D localization by 2*[eq.1]+[eq.2], or 3D localization by 2*[eq.1]+2*[eq.3], the latter with improved accuracy due to redundant measurements.

(41) Additionally according to said first, second and third embodiments of the present invention, when the beacon detects 3 or more GNSS satellites, resolves the navigation equations and acquires the precise GPS TIME, i.e. the 1PPS signal, the beacon is configured to transmit further bursts in synchronization with UTC. As appreciated by a skilled person, UTC and GPT TIME are synchronized to same rising edge of the seconds tick.

(42) FIG. 4 illustrates TOA Measurement Synchronized with GPS Clock. The diagram at the upper part of the picture shows the transmitted beacon signal vs. GPS TIME and the diagram at the bottom part of the picture shows the detection of said beacon signal (relayed by a satellite, as shown in FIG. 5) at the MEOLUT, again vs. same GPS TIME scale. Both the beacon and MEOLUT are shown to simultaneously detect the 1PPS pulse of the GPS, and the beacon is shown to transmit exactly at one of these 1PPS instants. The MEOLUT detection time is referred to the last 1PPS pulse, and the time difference equals to the distance travelled between beacon and MEOLUT divided by the speed of light.

(43) The present invention discloses also a radio beacon for localization via satellites, comprising: a receiver coupled to a transmitter; said receiver configured to record detection time of a first signal broadcast from a first satellite payload at a known epoch; said transmitter configured to transmit a second signal a predefined delay after said detection time, enabling a second satellite payload to relay said second signal to a remote receiver; and enabling at said remote receiver expressing the location coordinates of said beacon based on: said first and second satellite payloads position and said remote receiver position, said epoch, said predefined delay and the time at which said relayed second signal is detected at said remote receiver.

(44) FIG. 9 illustrates the Beacon Block Diagram according to the present invention, comprising two main blocks: a GPS receiver and a UHF transmitter. The GPS receiver is depicted at the left upper part, coupled to a GPS (L-band) antenna, and providing 3 outputs: position (Lat/Lon) and two timing signals: the PRN correlation pulse and the 1 PPS/1 KPPS pulse, possibly configurable on the same terminal. There are many GPS receivers in the market in form of components/modules/chip-sets that could be embedded in the beacon, as the skilled person appreciates, which commonly output the position and 1 PPS/1 KPPS data. Yet, the PRN correlation pulse is not a standard output in the industry, though typically is internally generated, and might be configured as an output in certain platforms such as software GNSS receivers.

(45) The transmitter block is framed by a dashed line, comprising 5 sub blocks: TCXO (Temperature Compensated Crystal Oscillator), micro-processor, PSK modulator, RF (Carrier frequency) generator and power amplifier. The TCXO is the master clock of the beacon, generating a basic frequency of 12.68875 MHz which is multiplied or divided to generate the RF frequency of 406.040 MHz and bit rate of 400 bps to clock the standard 144 bits message communicated in every burst. The processor generates that message and controls the beacon timing. The PSK modulator transforms the digital bits to PSK (Phase Shift Keying) signals modulating the RF carrier, amplified to 5 watts by the power amplifier and coupled to the UHF antenna. The PRN correlation pulse is routed to the PSK modulator via a T.sub.1 delay block to show that it controls the timing of the PSK output, i.e. timing of burst transmission, yet practically this can be done through the micro-processor, and the same applies to the 1 PPS/1 KPPS signal output from the GPS receiver. In some receivers, such as u-blox M8M, there is a general time pulse output port that can be configured to 1 PPS, 1 KPPS and likewise.

(46) The transmitter sub-blocks (micro-processor, modulator, RF generator, power amplifier and TCXO) can be implemented with off the shelf components, known to those skilled in the art. The processor, which typically comprises also RAM, EPROM and peripherals, sometimes referred to as micro-controller, runs a dedicated real time software, such that is well practiced in the art.

(47) According to a fourth embodiment of the present invention, the beacon is a Maritime Survivor Location Device (MSLD), in form of a wrist watch, particularly addressing Man over Board (MOB) situations. The beacon, designed following the block diagram presented in FIG. 9, is activated upon falling overboard, either manually or triggered by water sensing or impact sensing devices embedded in the beacon. Upon activation, the GPS receiver is turned on, searching for GPS satellites, and the processor is prepared to broadcast periodic bursts indicating the distress situation and parameters. These parameters typically include a unique ID and the position as acquired at the beacon by an embedded GNSS receiver. Since the GPS receiver is typically turned on in cold start, i.e. with no valid position and time, it can take up to 30-40 seconds of continuously monitoring some satellites until the position is fixed, and this could be very problematic if the device antenna drastically changes its orientation and immersed in the water from time to time, as expected from a wrist-worn device in such situation. Nevertheless, a single GPS satellite might be momentarily detected, since it requires less than a second to acquire the PRN which is continuously broadcast by every GPS satellite, repeatedly every 1 ms. Then, the GPS receiver outputs the PRN correlation pulse, monitored by the processor which accordingly configures the timing of the next burst.

(48) Further according to this fourth embodiment, said first signal, i.e. the GNSS signal, is transmitted repeatedly at a substantially fixed period, and said radio beacon configured to dynamically select at least one of said first signals detection time after which said second signal is transmitted, enabling a remote receiver to express the location coordinates of the MSLD up to a certain ambiguity, even if said dynamic selection is unknown at the remote receiver. As appreciated by those skilled in the art, the GNSS signals comprise a PRN sequence that is repeatedly transmitted every 1 ms, and a navigation message structured with frames divided to subframes and further divided to words; each subframe transmitted every 6 s, aligned with the PRN timing; each subframe beginning with a Telemetry Word (TLM), enabling the receiver to detect the beginning of a subframe and determine the GNSS receiver clock time at which the navigation subframe begins; next is the handover word (HOW), specifying the time at which the first bit of the next subframe will be transmitted. Each of the TLM and HOW words contain 30 bits, each bit transmitted at 50 bps, also aligned with the PRN timing. This information can be used by the MSLD, embedded with a GNSS receiver, to generate a local replica of the satellite clock, though not at the same phase, but at any desired rate, as a reference to the GNSS epoch.

(49) At the remote receiver a same rate clock can be configured, this synchronized with the satellite clock, enabling using [eq.1]. If this clock rate is relatively high, e.g. 1 KPPS, the remote receiver cannot clearly determine at which phase of that clock the first signal was transmitted, so [eq.1] could be resolved just up to a certain ambiguity.

(50) Considering that ambiguity, this enables the remote receiver expressing the location coordinates of the beacon based on a TOA measurement according to the present invention, and further determine the beacon location based on further TOA and/or FOA measurements, according to the above expressed [eq.1], [eq.2], [eq.3] and [eq.5]. In particular, the beacon enables localization by a remote receiver upon a single burst transmission and single satellite in view.

(51) Further bursts emitted by the beacon, and/or additional satellites in view relaying these bursts, enable reducing the localization error.

(52) The present invention further discloses a MEOLUT for localization of radio beacons, comprising an RF module and a processor; said RF module configured to detect signals transmitted from satellites; said processor configured to express the coordinates of a beacon emitting signals relayed by satellites, based on: a time at which a first satellite payload transmits a first signal and position thereof; a predefined delay at which said beacon transmits a second signal after detecting said first signal; position of a second satellite payload relaying said second signal; detection time of said relayed second signal and position thereof.

(53) FIG. 10 illustrates the MEOLUT Block Diagram according to COSPAS-SARSAT MEOLUT PERFORMANCE SPECIFICATION AND DESIGN GUIDELINES C/S T.019 Issue 1 Dec. 2015. The MEOLUT is divided to five main blocks: Antennas RF subsystems, Antenna management, Reception, Processing, and MEOLUT management; two external interfaces are also shown: to Networked MEOLUT(s) and to MCC.

(54) The Antennas RF subsystems module is responsible to detect the satellite signals, both GNSS and SAR relayed signals, and the Antenna management module controls the operation thereof; the Reception module gets the weak and noisy RF signals detected at the Antennas and preprocesses the signals typically through amplification, filtering, mixing, sampling, digitizing and so on, to a level and format manageable by the Processing module; at the Processing module the TOA and FOA measurements are employed, as well as the beacon localization, controlled by the MEOLUT management module; the beacons localization results, and complementary data are reported to the MCC (Mission Control Centre); optionally, raw measurement data is reported to and received from other MEOLUTs in order to enhance the localization performance.

(55) Typically, the Processing module will be responsible to express the beacon coordinates and resolve the beacon position based on TOA and FOA measurements according to the present invention. Preferably, most of these tasks are performed in software, on a powerful processor, running at high speed and using large RAM and ROM space.

(56) According to a fifth embodiment of the present invention, the MEOLUT is actually a ROMEOLUTRoaming MEOLUT, i.e. mobile remote receiver, for tracking beacons on the move from aircraft, ships, terrestrial vehicles and even carried by persons, in search and rescue operations. The ROMEOLUT according to this fifth embodiment of the present invention is based on a sub-set of the blocks depicted in FIG. 10: Antennas RF subsystems, Reception and Processing; the Antennas RF subsystems and the Reception module are configured to detect the GNSS satellite signals, but SAR signals directly from the beacon, i.e. directly detect the second signal (not the relayed); preferably, two separate antennas are used: an L-band GNSS antenna and a UHF antenna, and two receivers: a GNSS receiver and a UHF PSK receiver, the latter could be implemented based on one of Analog Devices: ADRF6850 (100 MHz to 1000 MHz Integrated Broadband Receiver) or ADRF6806 (50 MHz to 525 MHz Quadrature Demodulator with Fractional-N PLL and VCO).

(57) The Processing module could be implemented based on, for example, a Texas Instruments MSP430 controller, possibly with on an ARM core, to run the TOA and FOA measurements, as well as the beacon localization; since the ROMEOLUT is configured to directly detect the beacon bursts (second signals) the second satellite payload is redundant and in [eq.1] and [eq.5] the position coordinates of the second payload satellite are united with the coordinates of the beacon. This version of [eq.1] relevant to the fifth embodiment is therefore:
C*(tr.sub.mtx.sub.eT.sub.1)=[(xx.sub.1).sup.2+(yy.sub.1).sup.2+(zz.sub.1).sup.2]+[(x.sub.mx).sup.2+(y.sub.my).sup.2+(z.sub.mz).sup.2];[eq.6]:

(58) Further according to the fifth embodiment, the first signal is a GNSS signal transmitted repeatedly at a fixed period, and the beacon dynamically selects some of said GNSS signals detection time after which a burst is transmitted, then the ROMEOLUT is further configured to express the location coordinates of the beacon up to a certain ambiguity, even if said dynamic selection is unknown. Naturally, since the ROMEOLUT is configured to detect the beacon bursts directly, the typical operation range of the ROMEOLUT is much shorter compared to a standard MEOLUT served by SAR satellites, thus the ambiguity in [eq.6] due to miss interpretation of tx.sub.e and/or T.sub.1 at the ROMEOLUT can be removed much easier.

(59) The true scope the present invention is not limited to the presently preferred embodiments disclosed herein. For example, the foregoing disclosure uses explanatory terms, such as GPS, GNSS as well as radio beacon, MEOLUT, and the Cospas-Sarsat system, which should not be construed so as to limit the scope of protection of the claims, or to otherwise imply that the inventive aspects of the disclosed methods and devices are limited to the particular methods and apparatus disclosed.

(60) In many cases, the place of implementation described herein is merely a designer's preference and not a hard requirement. For example, functions disclosed as implemented at the detecting device may alternatively be partially implemented at satellite payloads, or vice versa. Also, functions or blocks described as implemented in hardware might be alternatively implemented in software, or vice versa. Given the rapidly declining cost of digital signal processing and other processing functions, it is easily possible, for example, to transfer the processing or a particular function from one of the functional elements described herein to another functional element without changing the inventive operation of the system.